Polytrimethylene ether glycol and its uses have been described in the art. It can be prepared by dehydration of 1,3-propanediol or by ring opening polymerization of oxetane, typically using an acid catalyst.
For water based applications such as coatings, and personal care products, it is desirable to produce short chain or low molecular weight polytrimethylene ether glycol from the polycondensation of 1,3-propanediol. However, low molecular weight polytrimethylene ether glycol prepared by dehydration of 1,3-propanediol monomer can contain unreacted monomer in significant amounts, e.g., about 8 to 15% by weight, which is undesirable for certain applications, and removal of the monomer from the product can make the product expensive. Also, when sulfuric acid is used as a catalyst, a substantial portion of the acid is converted to the ester, alkyl hydrogen sulfate, during polycondensation and can affect the hydroxy functionality of the polymer and thereby limit its use as a reactive intermediate.
In addition, lower molecular weight polymers are more water soluble than the polymer having higher molecular weights. For this reason, it can be difficult to implement the hydrolysis step in purification of lower molecular weight polymers, because of the equilibrium reaction between the sulfate ester and glycol end groups, and therefore difficult to achieve a distinct aqueous and organic phase separation. Also, the water washing steps utilized in conventional processes can be a substantial disadvantage, because the water washing not only removes the acid present but also removes water-soluble short polyether chains. Furthermore, recovery of the soluble fraction of the polymer from aqueous solutions is desirable in order to achieve desirably high polymer yields, which can be expensive and time consuming because it requires distillation of large amounts of water and can lead to undesirably high capital, maintenance, and operating costs.
It is therefore desirable to manufacture relatively low molecular weight and water soluble polytrimethylene ether glycol free of acid catalyst and acid catalyst derived end groups (e.g. acid ester end groups) and containing less than 2.0% by weight unreacted propanediol. It is further desirable to prepare such polytrimethylene ether glycol using acid-catalyzed polymerization without the hydrolysis and/or water washing steps. The present invention is directed to these and other ends.